Noise shaping denotes a method in which noise is concentrated to a greater extent in specific frequency ranges and as a result the noise energy in the frequency spectrum is shifted in a manner that is advantageous for the application. Noise shaping can be carried out by means of so called modulators/noise shapers.
Conventional implementations of digital noise shapers or of analog noise shapers such as are used in analog-to-digital converters (ADCs), for example, exhibit strong interference signals for low input signals, in the case of microphones so called idle tones, which can occur for example at half the sampling frequency of a digitally converted analog signal.
Digital microphones have to provide a 1-bit output stream for many applications, which output stream can be provided by a digital modulator.
By way of example, in stereo applications, an intermodulation of the idle tones can occur, such that an interference signal can occur in the audio band, that is to say in the range audible to human hearing.
In the case of mono applications, too, idle tones can occur, in particular in the case of high loads.
Comparable interference signals can also occur in some applications of analog-to-digital converters.
A conventional procedure in such cases consists in superposing a so called “dither” signal with a signal input of a quantizer of an analog-to-digital converter or modulator used.
In this case, the dither signal can be a periodic or a (pseudo) random signal, which can be realized for example by means of a linear feedback shift register (LFSR). This has the effect that the signal-to-noise ratio (SNR) is improved by the noise added by means of the dither signal in a specific frequency range.
In the microphone application, however, a digital noise shaper is required for generating the abovementioned 1 bit output stream and the conventional approaches are inadequate; employing the procedures mentioned above would lead to an impediment of the signal-to-noise ratio in some cases.